The present invention relates to a noise reduction apparatus at the time of an audio signal receiving, and more specifically to a noise reduction apparatus used for, for example, a car radio in which a pulsive noise is easily mixed, and to an audio output apparatus.
For example, when the electromagnetic noise in the circumstance of the car is considered, various pulsive electromagnetic noises (sometimes called the pulsive noise) such as an ignition noise, or mirror noise, are generated. Because these pulsive noises mix into a reception antenna connected to a car radio inside the car, it is ordinarily often experienced that the pulsive noise is generated in an output audio signal, therefore, generally for the car radio, the noise reduction apparatus to remove the pulsive noise is used.
FIG. 8 is a block diagram of the conventional (pulsive) noise reduction apparatus disclosed in, for example, JP-A-63-87026. In the drawing, when an FM intermediate frequency signal of an FM receiver is inputted, a detection signal outputted from an FM detection circuit 1 is supplied to a delay circuit 2 composed of a LPF (low pass filter) and delayed, and the output of the delay circuit 2 is supplied to a stereo demodulation circuit 5 through a gate circuit 3, and a level hold circuit 4. Further, the detection signal is supplied to a HPF (high pass filter) 6, and a noise component signal passed through the HPF 6 is amplified by a noise amplifier 7, and supplied to a noise detection circuit 8.
The noise detection circuit 8 is composed of a rectifier circuit to rectify the output signal of the noise amplifier 7, and a noise detection output is obtained thereby. This noise detection output is supplied to a waveform shaping circuit 9 and an integration circuit 10. Incidentally, a noise detection means 11 is structured by including the HPF 6, noise amplifier 7, noise detection circuit 8, waveform shaping circuit 9 and integration circuit 10.
The waveform shaping circuit 9 transforms the noise detection output into a pulse having a pulse width of a predetermined time period width, and supplied to the gate circuit 3. The gate circuit 3 is driven by a pulse supplied from the waveform shaping circuit 9 to the gate circuit 3 and comes to a signal cut out condition, and at the time of the signal cut out condition, a delay output level before the signal cut out is held by the level hold circuit 4, and supplied to the stereo demodulation circuit 5.
According to this, the generation of a spike noise by the sudden change of the potential of the demodulation signal due to the pulsive noise is prevented. When the pulse is not supplied from the waveform shaping circuit 9, the gate circuit 3 and the level hold circuit 4 become the signal through condition (through).
Further, the integration circuit 10 smoothes the noise detection output and obtains a DC signal corresponding to the noise level, and supplies the output of the integration circuit 10 to the noise amplifier 7 (feedback), and thereby, an AGC loop is formed.
Incidentally, the delay circuit 2 is provided to make up for a time period from the time when the pulsive noise is supplied to the HPF 6 to the time when the gate circuit 3 is made to be in the cut out condition. Further, because, in the stereo demodulation circuit 5, as shown in FIG. 9, the Lch (left channel) signal and Rch (right channel) signal are inputted in the form which is balancedly modulated with the frequency of 38 kHz around (Lch+Rch)/2, the LCh signal and the Rch signal can be separately picked out by the time division with, for example, 38 kHz.
Further, there is also a method by which the signal is corrected by an average value from the levels before and after the pulsive noise is generated, other than a method by which the former signal is level-held and outputted, as described above. Incidentally, in this method, the following problem exists.
In FIG. 10A, a waveform in the case where a correction error in which a low frequency signal is corrected to a correction period, becomes maximum, is shown. In the drawing, ◯ mark is a value in which ● mark is corrected, and the difference between ◯ mark and ● mark shows the correction error.
Next, in FIG. 10B, a waveform when a high frequency signal is corrected to a correction period, is shown. In the drawing, ◯ mark shows a value in which ● mark is corrected. In the same manner as in FIG. 10A, the difference between ◯ mark and ● mark shows the correction error.
Herein, when each of correction errors is observed, FIG. 10B is larger. That is, it is found that the relative relationship of the time width of the frequency to the correction period is very important, and even when a signal of a high frequency component is corrected, the error is large. Accordingly, even when the correction is conducted on the signal of the high frequency component, the correction error is heard as a noise. Herein, in contrast to that the pulse width of the pulsive noise is several tens μs to several hundreds μs, a composite signal has, as shown in FIG. 9, a component which is balancedly modulated with 38 kHz, and because the period of the signal is shorter than that of the pulsive noise, the correction error as shown in FIG. 10B, is generated.
A case where the pulsive noise is generated in a time period of ta of the demodulated signal in the FM demodulator 1, is shown in FIG. 19A. A value of A in FIG. 19A is held in a period of ta in the level hold circuit 4.
A Lch waveform obtained by the stereo demodulation of this corrected signal is shown in FIG. 19B, and a Rch waveform is shown in FIG. 19C.
In this case, as shown in FIG. 19B, because the Lch is held at the level of the Lch, the Lch after the stereo demodulation can be exactly corrected, however, the Rch is, as shown in FIG. 19C, held at the level of the Lch, when the difference between the Rch and Lch is large, the large correction noise as shown in FIG. 19C, is generated.